"We don't know who did it," Middletown Police Officer William Hertler said Monday. "The train claims no permission to the sign, and the state said it had nothing to do with it."

Officials said it is possible the sign was put up as a prank.

"It looks like it was an unknown third party," Hertler said. "It seems like an elaborate joke."

Middletown Mayor Dan Drew said he assumed it was part of the Department of Homeland Security campaign but has not received any details from the department.

A spokesman for the Providence-Worcester Railroad, who owns the local railroad tracks, said he is unaware of the signs on the bridge. The railroad company does not patrol the tracks, but if it sees something unusual it usually informs the local police.